1. Field of the Invention
This invention relates generally to the field of motion capture. More particularly, the invention relates to an improved apparatus and method for tracking and capturing the motion and/or expression of a performer.
2. Description of the Related Art
“Motion capture” refers generally to the tracking and recording of human motion. Motion capture systems are used for a variety of applications including, for example, video games and computer-generated movies. In a typical motion capture session, the motion of a “performer” is captured and translated to a computer-generated character.
As illustrated in FIG. 1, in a motion capture system, a plurality of motion tracking markers 101-116 are attached at various points on a performer's body. The points are selected based on the known limitations of the human skeleton. For example, markers 107 and 114, attached to the performer's knees, represent pivot points for markers 115 and 116, attached to the performer's feet. Similarly, markers 104 and 111, attached to the performer's elbows, represent pivot points for sensors 105 and 112, attached to the performer's hands.
Different types of motion capture systems have been developed over the years. For example, in a “magnetic” motion capture system, the motion markers attached to the performer are active devices that measure their position in a magnetic field enveloping the performer. By contrast, in an optical motion capture system, such as that illustrated in FIG. 1, the motion markers 101-116 are comprised of retro-reflective material, i.e., a material which reflects light back in the direction from which it came, ideally over a wide range of angles of incidence. Two or more cameras 120, 121, 122 are positioned to capture the light reflected off of the retro-reflective markers 101-116.
A motion tracking unit 150 coupled to the cameras is programmed with the relative position of each of the markers 101-116 and the known limitations of the performer's body. For example, if the relationship between motion sensor 107 and 115 is programmed into the motion tracking unit 150, the motion tracking unit 150 will understand that sensor 107 and 115 are always a fixed distance apart, and that sensor 115 may move 107 within a specified range. These constraints allow the motion capture system to usually be able to identify each marker distinctly from the other and thereby know which part of the body each marker's position is identifying. The markers don't actually identify any body parts, strictly their own position and indentity. Also, once the markers are identified individually, the motion capture system is able to determine the position of the markers 101-116 via triangulation between multiple cameras (at least 2) that see the same marker. Using this information and the visual data provided from the cameras 120-122, the motion tracking unit 150 generates artificial motion data representing the movement of the performer during the motion capture session.
A graphics processing unit 152 renders an animated representation of the performer on a computer display 160 (or similar display device) using the motion data. For example, the graphics processing unit 152 may apply the captured motion of the performer to different animated characters and/or to include the animated characters in different computer-generated scenes. In one implementation, the motion tracking unit 150 and the graphics processing unit 152 are programmable cards coupled to the bus of a computer (e.g., such as the PCI and AGP buses found in many personal computers). One well known company which produces motion capture systems is Motion Analysis Corporation (see, e.g., www.motionanalysis.com).
One problem which exists with current motion capture systems, however, is that when the markers move out of range of the cameras, the motion tracking unit 150 may lose track of the markers. For example, if a performer lays down on the floor on his/her stomach (thereby covering a number of markers), moves around on the floor and then stands back up, the motion tracking unit 150 may not be capable of re-identifying all of the markers.
As such, after a performance, a significant amount of “clean up” is typically required during which computer programmers or animators manually identify each of the “lost” markers to the image tracking unit 150, resulting in significant additional production costs.
In addition, while current motion capture systems are well suited for tracking full body motion, current systems are ill-equipped for tracking the more detailed, expressive movement of a human face. For example, the size of the markers used in current systems allows for only a limited number of markers to be placed on a performer's face, and movement around the performer's lips and eyes, which are small but critical in expression, may be lost by the use of a limited number of markers.
Accordingly, what is needed is an improved apparatus and method for tracking and capturing the motion and/or expression of a performer.